Marin's already terrible northbound Highway 101 evening commute could get even more gnarly Friday if BART workers strike, according to transportation officials.

Though a 60-day cooling-off period ordered by Gov. Jerry Brown ends at 11:59 p.m. Thursday, ongoing negotiations keep hope alive that BART will avoid a strike stranding 200,000-plus daily riders, jamming freeways and bridges and leaving standing-room only on buses and ferries.

Commuters, parents driving children home from school and people taking local trips already produce virtual gridlock from the Richardson Bay Bridge to the Greenbrae Interchange during the evening commute on Highway 101.

Now imagine thousands of more cars using the highway and the Richmond-San Rafael Bridge to get back home to the East Bay.

"The impact in Marin during the last strike in July was quite muted, but that could be much different this time," said John Goodwin, spokesman for the Metropolitan Transportation Commission, the agency charged with making sure traffic flows smoothly on Bay Area highways. "Now there are kids back in school and that adds to the traffic."

Making matters worse is ongoing road deck work on the Richmond side of the Richmond-San Rafael Bridge. In recent weeks that has served to back up traffic across the span to Marin and Highway 101.

"That could exacerbate the already heavy traffic in the evening in Marin," Goodwin said.

But a BART shutdown appeared increasingly unlikely as the unions, who customarily give riders at least 72 hours' warning of a walkout, again declined Wednesday to issue a strike notice and said they are instead focused on reaching a deal.

Still, it would be within the unions' power to shut down the system Friday morning if a deal is not reached before then — an option they have not ruled out.

If there is a strike, check alert.511.org for commuting options. If your only choice is to hit the road, heed this advice: "Get there early," said Goodwin said.

Columbus Day is Monday, and that could lead some people to take a four-day weekend and avoid the hassles sure to come if BART service is suspended.

BART workers last went on strike July 1-5, when 52 percent of the 56,000 commuters who ordinarily ride BART between the East Bay and San Francisco in the morning either worked from home or canceled discretionary trips across the bay. And 12,000 carpooled with co-workers, friends or joined casual carpools, catching a ride with a stranger to take advantage of carpool lanes and lower bridge tolls.

But ridership now is 30 percent higher than it was in July because fewer people are on vacation, so a strike this time of year would have a bigger impact.

"Yes, it will be bad," said Greenbrae commuter Susan Sherman via email. "All the East Bay will need to get to the city. They all think they are so clever to take the Richmond Bridge. Ugh."

Contact Mark Prado via email at mprado@marinij.com. The Bay Area News Group contributed to this report.